This study utilizes the indirect fluorescent antibody technique (IFA) to determine the presence and clinical significance of pre-formed antibody to vascular endothelium in renal transplantation patients and correlates the presence of this antibody with lymphocytotoxicity and renal cell cytotoxicity reactions. Studies are undertaken to determine the specificity of this antibody and improve the reliability and sensitivity of its detection, and to compare the sensitivity and reliability of the anti-globulin lym phocytotoxicity reaction with the IFA reaction to vascular endothelium in predicting accelerated graft rejection.